Heretofore, in producing cellulose acylate films for use in liquid-crystal image display devices, a solution-casting method has been principally carried out, which comprises dissolving cellulose acylate in a chlorine-containing organic solvent such as dichloromethane, casting it on a substrate, and drying it to form a film. Dichloromethane, a type of a chlorine-containing organic solvent has been favorably used as a solvent for cellulose acylate, since it is a good solvent for cellulose acylate and has a low boiling point (about 40° C.), therefore having the advantage of easy vaporization in a film formation step and in a drying step.
Recently, however, from the viewpoint of environmental protection, it has become strongly required to retard release of chlorine-containing organic solvent and other organic solvents. Accordingly, some countermeasures have heretofore been tried and employed for retarding the release of organic solvent as much as possible, for example, by using a more severely-controlled closed system enough to prevent the leakage of organic solvent from it, or by leading the organic solvent, if any, leaked out from a system into a gas absorption column to adsorb it before the organic solvent is released in outdoor air, or by burning the organic solvent with flames, or by decomposing it with electron beams. Even by these countermeasures, however, it is still impossible to completely prevent the release of organic solvent, and further improvements are required.
Accordingly, as a film forming method of cellulose acylate not using a chlorine-based organic solvent, a method of producing a film by forming a film out of a solution using a dope in which cellulose acylate is dissolved in a non-chlorine-based organic solvent containing alcohol having 1 to 4 carbons has been disclosed (see Patent Document 1). According to the method, at least the non-emission of the chlorine-based organic solvent can be accomplished.
A melt-casting method of producing a film of a specific cellulose acylate has been proposed as a film formation method not using an organic method (Patent Document 1). According to the method, the carbon chain of the ester group in cellulose acylate is prolonged so as to lower the melting point of the polymer for easy melt-casting film formation of the polymer. Concretely, cellulose acetate is changed into cellulose propionate or cellulose butyrate, thereby enabling melt-casting film formation of the polymer. We, the present inventors tried forming a polarizer, using a film produced according to the melt-casting film formation method described in this patent document, and tried building the polarizer in a liquid-crystal display device, but we knew formation of blurry images. Accordingly, the improvement was desired.    Patent Document 1: JP-A-2000-344904    Patent Document 2: JP-A-2000-352620